Plasma ashing apparatus and endpoint detection process

ABSTRACT

A plasma ashing apparatus for removing organic matter from a substrate including a low k dielectric, comprising a first gas source; a plasma generating component in fluid communication with the first gas source; a process chamber in fluid communication with the plasma generating component; an exhaust conduit in fluid communication with the process chamber; wherein the exhaust conduit comprises an inlet for a second gas source and an afterburner assembly coupled to the exhaust conduit, wherein the inlet is disposed intermediate to the process chamber and an afterburner assembly, and wherein the afterburner assembly comprises means for generating a plasma within the exhaust conduit with or without introduction of a gas from the second gas source; and an optical emission spectroscopy device coupled to the exhaust conduit comprising collection optics focused within a plasma discharge region of the afterburner assembly. An endpoint detection process for an oxygen free and nitrogen free plasma process comprises monitoring an optical emission signal of an afterburner excited species in an exhaust conduit of the plasma asher apparatus. The process and apparatus can be used with carbon and/or hydrogen containing low k dielectric materials.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of the legally related U.S.Non-Provisional application Ser. No. 10/249,964, filed May 22, 2003,which is fully incorporated herein by reference.

BACKGROUND

The present disclosure relates to semiconductor apparatuses andprocesses, and more particularly, to plasma mediated processes andplasma apparatuses suitable for ashing organic material from a substrateincluding a low k dielectric material.

Recently, much attention has been focused on developing low k dielectricthin films for use in the next generation of microelectronics. Asintegrated devices become smaller, the RC-delay time of signalpropagation along interconnects becomes one of the dominant factorslimiting overall chip speed. With the advent of copper technology, R hasbeen pushed to its practical lowest limit for current state of the artso attention must be focused on reducing C. One way of accomplishingthis task is to reduce the average dielectric constant (k) of the thininsulating films surrounding interconnects. The dielectric constant (k)of traditional silicon dioxide insulative materials is about 3.9.Lowering the dielectric constant (k) below 3.9 will provide a reducedcapacitance.

Low k dielectric materials used in advanced integrated circuitstypically comprise organic polymers or oxides and have dielectricconstants less than about 3.5. The low k dielectric materials can bespun onto the substrate as a solution or deposited by a chemical vapordeposition process. Important low k film properties include thicknessand uniformity, dielectric constant, refractive index, adhesion,chemical resistance, thermal stability, pore size and distribution,coefficient of thermal expansion, glass transition temperature, filmstress, and copper diffusion coefficient.

In fabricating integrated circuits on wafers, the wafers are generallysubjected to many process steps before finished integrated circuits canbe produced. Low k dielectric materials, especially carbon containinglow k dielectric materials, can be sensitive to some of these processsteps. For example, plasma used during an “ashing” step can strip bothphotoresist materials as well as remove a portion of the low-kdielectric film. Ashing refers to a plasma mediated stripping process bywhich photoresist and post etch residues are stripped or removed from asubstrate upon exposure to the plasma. The ashing process generallyoccurs after an etching or implant process has been performed in which aphotoresist material is used as a mask for etching a pattern into theunderlying substrate or for selectively implanting ions into the exposedareas of the substrate. The remaining photoresist and any post etch orpost implant residues on the wafer after the etch process or implantprocess is complete must be removed prior to further processing fornumerous reasons generally known to those skilled in the art. The ashingstep is typically followed by a wet chemical treatment to remove tracesof the residue, which can cause further degradation of the low kdielectric, loss of material, and may also cause increase in thedielectric constant.

It is important to note that ashing processes significantly differ frometching processes. Although both processes may be plasma mediated, anetching process is markedly different in that the plasma chemistry ischosen to permanently transfer an image into the substrate by removingportions of the substrate surface through openings in a photoresistmask. The plasma generally includes high-energy ion bombardment at lowtemperatures and low pressures (on the order of milli-Torrs) to removeportions of the substrate. Moreover, the portions of the substrateexposed to the ions are generally removed at a rate equal to or greaterthan the removal rate of the photoresist mask. In contrast, ashingprocesses generally refer to selectively removing the photoresist maskand any polymers or residues formed during etching. The ashing plasmachemistry is much less aggressive than etching chemistries and isgenerally chosen to remove the photoresist mask layer at a rate muchgreater than the removal rate of the underlying substrate. Moreover,most ashing processes heat the substrate to temperatures greater than200° C. to increase the plasma reactivity, and are performed atpressures of about 1.0 Torr. Thus, etching and ashing processes aredirected to removal of significantly different materials and as such,require completely different plasma chemistries and processes.Successful ashing processes are not used to permanently transfer animage into the substrate. Rather, successful ashing processes aredefined by the photoresist, polymer and residue removal rates withoutaffecting or removing underlying layers, e.g., low k dielectric layers.

Studies have suggested that a significant contribution to low kdielectric degradation during photoresist removal processes results fromthe use of, oxygen and/or nitrogen and/or fluorine containing gassources typically used for ashing. Although gas mixtures containing oneor more of these sources efficiently ash photoresist from the substrate,the use of these gas sources has proven detrimental to substratescontaining low k dielectrics. For example, oxygen-containing plasmadischarges are known to raise the dielectric constant of low kdielectric underlayers during plasma processing. The increases indielectric constant affects, among others, interconnect capacitance,which directly impacts device performance. Moreover, the use ofoxygen-containing plasma discharges is generally less preferred foradvanced device fabrication employing copper metal layers since coppermetal is readily oxidized at the elevated temperatures typicallyemployed for photoresist ashing. Occasionally, the damage is notdetected during metrology inspection of the substrate after plasmaprocessing. However, the damage can be readily demonstrated by asubsequent wet cleaning process, as may be typically employed afterplasma ashing, wherein portions of the carbon and/or hydrogen-containinglow k dielectric material are removed. The removed portions of thedielectric material are a source of variation in the critical dimension(CD) of the feature that is frequently unacceptable and impacts overalldevice yield. Moreover, even if a wet clean process is not included, theelectrical and mechanical properties of the dielectric material may bechanged by exposure to the oxygen-free plasma discharges therebyaffecting operating performance. It is believed that carbon is depletedfrom the dielectric material during the plasma exposure.

Ideally, the ashing plasma should not affect the underlying low kdielectric layers and preferably removes only the photoresist material.The use of SiO₂ as the dielectric material provided high selectivitywith these gas sources. In order to minimize damage to the low kdielectric, oxygen and nitrogen free plasma processes have beendeveloped. One such process includes generating plasma from a gasmixture comprising helium and hydrogen. However, the mechanism ofremoval is different for these less aggressive plasma discharges. Theoxygen and nitrogen free plasma such as the plasma formed from heliumand hydrogen does not ash the photoresist in the traditional sense.Rather, it is believed that the plasma causes portions of thephotoresist to sublime from the substrate. As a result of the mechanismof removal, while effective for removing photoresist material from thesubstrate, the plasma exposure tends to deposit large bodies of thesublimed photoresist and byproducts within the processing chamber and inareas downstream from the plasma process chamber such as in the throttlevalve and exhaust lines. The buildup of these ashing materials can leadto short mean-time-between-clean (MTBC) times and frequentrebuild/replacement of vacuum hardware resulting in loss of throughputand increased costs of ownership. Additionally, deposits of photoresistmaterial within the process chamber that are located above the plane ofthe substrate can lead to particulate contamination on the substrate,thereby further affecting device yields.

An additional problem with oxygen free and nitrogen free plasmadischarges is the non-uniformity of the plasma exposure. Since theseplasma discharges are less aggressive, non-uniformity is a significantissue. Some downstream plasma ashers have a narrow diameter orificeplasma tube in which the plasma is generated. The diameter of thesubstrate is generally much larger than the diameter of the plasma tubeorifice. As such, baffle plates are typically positioned near the plasmatube outlet to deflect the plasma as it enters the process chamber suchthat the plasma species in the plasma are uniformly dispersed across thesubstrate. However, it has been found that the less aggressive plasmadischarges have fewer reactive species and the dispersal from the centerpoint of the baffle plate to its outer edge can result in hot spots onthe wafer, i.e., areas of non-uniformity. For example, it has beenspeculated that hydrogen radicals generated within a plasma recombine asthe hydrogen species travel from the center most impingement point onthe baffle plate in the axial flow reactor to the outer edges of thebaffle plate, thereby leading to lower ashing rates at the edge of thewafer. In chamber designs where the diameter of the wafer is comparableto that of the plasma tube, non-uniformity of radicals can be mitigatedin other ways.

Another problem with oxygen free and nitrogen free plasmas concernsendpoint detection. Traditional endpoint detection methods and apparatusare not suitable for muse with these types of plasma discharges. Forexample, as in the case of plasma formed from a hydrogen and helium gasmixture, no optically excited species are created at the wafer planethat generate a signal suitable for endpoint detection.

Accordingly, there remains a need for improved processes and apparatusesfor generating oxygen and nitrogen free plasma discharges for use withlow k dielectrics.

BRIEF SUMMARY

Disclosed herein is a plasma ashing apparatus for removing photoresistand/or post etch residues from a substrate, comprising a first gassource; a plasma generating component in fluid communication with thefirst gas source, wherein the plasma generating component generates afirst plasma for selectively removing the photoresist and/or post etchresidues from the substrate; a process chamber in fluid communicationwith the plasma generating component for receiving the plasma, whereinthe process chamber contains the substrate; an exhaust conduit in fluidcommunication with the process chamber; wherein the exhaust conduitcomprises a port for introducing a second gas source and an afterburnerassembly coupled to the exhaust conduit, wherein the port is disposedintermediate to the process chamber and the afterburner assembly; and anoptical detection system coupled to the exhaust conduit comprisingcollection optics focused within a plasma discharge region of theafterburner assembly.

In another embodiment, a downstream plasma ashing and/or residue removalapparatus comprises means for generating a plasma in an exhaust conduitin fluid communication with a process chamber; means for monitoring anemission signal for species generated within the plasma; and means fordetermining an endpoint of a plasma ashing and/or residue removalprocess on a substrate in the process chamber from the emission signalproduced in the exhaust conduit.

A method for detecting an endpoint for an oxygen free and nitrogen freeplasma ashing process, comprising exposing a substrate comprisingphotoresist material and/or post etch residues thereon to the oxygenfree and nitrogen free plasma in a process chamber; removing thephotoresist material and/or post etch residues from the substrate;exhausting the removed photoresist material and/or post etch residuesfrom the process chamber into an exhaust conduit fluidly coupled to theprocess chamber; selectively introducing an oxidizing gas into theexhaust conduit; generating an oxygen containing plasma from theoxidizing gas and the exhausted photoresist material and/or post etchresidues to form emissive species; and optically monitoring an emissionsignal produced by the emissive species to determine the endpoint of theoxygen free and nitrogen free plasma ashing.

In another embodiment, an endpoint detection process for an oxygen freeand nitrogen free plasma ashing process for removing photoresist and/orresidues from a substrate, comprising introducing an oxidizing gas and aplasma ashing discharge into an exhaust conduit of a plasma asherapparatus, wherein the plasma ashing discharge comprises photoresistmaterial, post etch residues, and post ashing products, and wherein theplasma ashing discharge is free from atomic nitrogen and atomic oxygenspecies; generating a plasma from the oxidizing gas and the plasmaashing discharge to form emissive species; and optically monitoring anemission signal intensity correlating to the emissive species, whereinan endpoint of the oxygen free and nitrogen free plasma ashing processis detected when the emission signal intensity correlating to theemissive species is no longer present.

In another embodiment, a method for determining an endpoint of an oxygenfee and nitrogen free plasma ashing process used for strippingphotoresist material from a substrate having a carbon containing low kdielectric material, comprising exposing the substrate to the oxygenfree and nitrogen free plasma ashing process in a process chamber toremove the photoresist material from the substrate and form volatilebyproducts; exhausting the photoresist material and volatile byproductsfrom the process chamber into an exhaust conduit; selectivelyintroducing an oxidizing gas into the exhaust conduit, wherein theoxidizing gas does not flow into the process chamber; generating aplasma in the exhaust conduit from oxidizing gas, the exhaustedphotoresist material, and the volatile byproducts; measuring an emissionsignal intensity in the exhaust conduit correlating to a wavelength ofabout 283 nm, 309 nm, about 387 nm, about 431 nm, about 434 nm, about468 nm, about 472 nm, about 513 nm, about 516 nm, about 656 nm, about777 nm, about 845 nm or a combination of at least one of the foregoingwavelengths; and determining the endpoint of the oxygen free andnitrogen free plasma ashing process in response to an observed change inthe emission signal within the exhaust conduit.

In yet another embodiment, a method for determining an endpoint of anoxygen fee and nitrogen free plasma ashing process used for strippingphotoresist material from a substrate having a carbon containing low kdielectric material, comprising generating a first plasma in a processchamber in the absence of oxygen and nitrogen from a gas mixturecomprising hydrogen or helium or a combination comprising at least oneof the foregoing gases; exposing the substrate provided in the processchamber to the first plasma to selectively remove photoresist materialand/or residues from the substrate; exhausting the removed photoresistmaterial and/or residues from the process chamber into an exhaustconduit;

generating a second plasma in the exhaust conduit to generate emissivespecies; and optically monitoring the emissive species, wherein anendpoint of the first plasma is detected when an intensity of theemissive species changes.

The above described and other features are exemplified by the followingfigures and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numberedalike in the several Figures:

FIG. 1 is a cross sectional view of a downstream plasma ashingapparatus; and

FIG. 2 shows a perspective view of a microwave enclosure for use in aplasma asher apparatus;

FIG. 3 shows a cross sectional view which schematically shows a plasmagenerating component suitable for use with the downstream plasma ashingapparatus;

FIG. 4 shows perspective view of the plasma ashing apparatus;

FIG. 5 is a partial cutaway perspective view of a photoresist asherprocess chamber into which is installed a gas distribution system;

FIG. 6 is a plan view of the gas distribution system in accordance withone embodiment;

FIG. 7 is a sectional view of the baffle plate assembly of FIG. 6, takenalong lines 6-6;

FIG. 8 is a plan view of the gas distribution system in accordance withanother embodiment; and

FIG. 9 is a graph illustrating a time evolution of a light intensityemitted for OH species generated from sublimated organic matter removedfrom a wafer due to heating the wafer to a temperature of 300° C. in anoxygen free and nitrogen free environment and oxidized in an exhaustconduit of a plasma ashing apparatus;

FIG. 10 is a graph illustrating a time evolution of carbon dioxidepartial pressure as measured with a residual gas analyzer in an oxygenfree and nitrogen free plasma and oxidized in an exhaust conduit of aplasma ashing apparatus;

FIG. 11 is a graph illustrating residual gas analysis of partialpressures for helium, nitrogen, and oxygen measured upstream from theafterburner and inlet for introducing oxygen gas into an exhaust conduitof a plasma asher apparatus (i.e., downstream from the wafer processchamber), wherein the flow rate of helium is varied;

FIG. 12 is a time evolution of optical signals showing variousphotoresist constituents (O and CN) and product (OH) produced uponstepwise heating of a photoresist-coated wafer;

FIG. 13 is a time evolution of a DUV photoresist removal process using ahydrogen/helium ashing process without employing an oxidizing gas in theendpoint monitoring process;

FIG. 14 is a time evolution of a DUV photoresist removal process using ahydrogen/helium ashing process and employing an oxidizing gas in theendpoint monitoring process; and

FIG. 15 is a time evolution of an I-line photoresist removal processusing a hydrogen/helium ashing process and employing an oxidizing gas inthe endpoint monitoring process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 generally illustrates an axial flow downstream plasma apparatus10 suitable for use in removing photoresist, sidewall deposits, and postetch residues from substrates including low k dielectric materials. Theplasma apparatus 10 generally comprises a gas delivery component 12, aplasma-generating component 14, a processing chamber 16, and an exhaustassembly component 18. The various components, in combination, provideimprovements in processing substrates with oxygen free and nitrogen freeplasma discharges, wherein the substrates include carbon containing lowk dielectric materials.

Downstream axial flow plasma apparatuses particularly suitable formodification in the present disclosure are plasma ashers, such as forexample, those microwave plasma ashers available under the trade nameFusion ES3LK and commercially available from Axcelis TechnologiesCorporation. Portions of the microwave plasma asher are described inU.S. Pat. Nos. 5,498,308 and 4,341,592, and PCT InternationalApplication No. WO/97/37055, herein incorporated by reference in theirentireties. As will be discussed below, the disclosure is not limited toany particular plasma asher in this or in the following embodiments. Forinstance, an inductively or capacitively coupled plasma reactor can beused.

Carbon-containing low k dielectrics are hereinafter defined as thosecarbon containing insulating materials suitable for use in themanufacture of integrated circuits or the like having a dielectricconstant less than about 3.5, with a dielectric constant less than about3.0 more preferred. The carbon-containing low k dielectric materials mayinclude pendant groups that contain carbon or may be carbon-containingwherein the backbone of the dielectric material is primarily comprisedof an interconnecting network of carbon. Carbon-containing low kdielectrics can generally be categorized as one of two types: organicand doped oxides. Examples of organic low k dielectric materials includepolyimides, benzocyclobutene, parylenes, diamond-like carbon,poly(arylene ethers), cyclotenes, fluorocarbons and the like, such asthose dielectrics commercially available under the trademarks SiLK, orBCB. Examples of doped oxide low k dielectric materials include methylsilsesquioxane, hydrogen silsesquioxanes, nanoporous oxides, carbondoped silicon dioxides, and the like, such as, for example, thosedielectrics commercially available under the trademarks CORAL, BLACKDIAMOND and AURORA. Both types of carbon-containing low-k materialsexist in dense and porous versions. Porous versions thereof arecommercially known under trademarks such as LKD, ORION, BOSS, or porousSiLK. Other carbon-containing low k dielectric materials will beapparent to one of ordinary skill in the art in view of this disclosure.

Likewise, hydrogen containing low k dielectrics are hereinafter definedas those hydrogen containing insulating materials suitable for use inthe manufacture of integrated circuits or the like having a dielectricconstant less than about 3.5. Many of the carbon containing low kdielectrics described above include one or more hydrogen atoms attachedto the carbon atoms within its chemical structure. However, suitablehydrogen containing low k dielectric materials in the present disclosureare not intended to be limited to carbon containing structures.

As shown in FIG. 1, the gas delivery component 12 preferably comprises agas purifier 20 in fluid communication with a gas source 22 (forgenerating the oxygen free and nitrogen free plasma) and a gas inlet 24of the plasma-generating component 14. An additional gas source (notshown) may be in fluid communication with the gas inlet 24 for providingin situ cleaning capabilities. In situ cleaning is a process forcleaning the process chamber 16 using the plasma source as opposed tomanually cleaning the chamber components by disassembling a portion ofthe process chamber for access thereto. In a preferred embodiment, thepurifier 20 is adapted to reduce the level of impurities to less thanabout 10 parts per million (ppm), with impurity levels less than about 5ppm more preferred, with impurity levels less than about 1 ppm morepreferred, and with less than about 100 parts per billion (ppb) mostpreferred. Suitable purifiers achieving these impurity levels includethose based on a metal gettering technology such as those gas purifierscommercially available under the trade name MONO TORR®. high flowpurifiers from SAES Pure Gas, Inc. The use of the gas purifier 20 influid communication with the gas source 22 employed for forming theplasma reduces the level of contaminants to amounts effective for robustprocessing of low k dielectric substrates, and in particular,carbon-containing low k dielectrics. Suitable gases for generating theoxygen free and nitrogen free plasma include, but are not intended to belimited to, hydrogen, helium, argon, neon, other inert gases,hydrocarbons, and combinations comprising one or more of the foregoinggases. For example, a helium gas source having a reported purity of99.999% can be undesirable for plasma mediated processing ofcarbon-containing low k dielectrics. Impurities such as H₂O, O₂, CO,CO₂, and N₂, can be at levels sufficient to cause erosion of the low kdielectric during further processing of the substrate and/ordeleteriously cause an increase in the dielectric constant. Preferably,the incoming gas for forming the plasma is purified to contain less than100 ppb of H₂O, O₂, CO, CO₂, and N₂. The purifier 20 is preferablyselected to be efficient and provide these preferred impurity levels atrelatively high flow rates, e.g., flow rates of about 100 to about12,000 standard cubic centimeters per minute (sccm) or more can beexpected for a 3.00 mm downstream plasma asher.

FIGS. 2 and 3 illustrate an exemplary microwave plasma-generatingcomponent 14 with which the present disclosure may be practiced. FIG. 4illustrates a perspective view of the plasma ashing apparatus 10including the microwave plasma-generating component and a perspectiveview of the process chamber 16. It is to be understood that theplasma-generating component 14 has been simplified to illustrate onlythose components that are relevant to an understanding of the presentdisclosure. Those of ordinary skill in the art will recognize that othercomponents may be required to produce an operational plasma ashingapparatus 10. However, because such components are well known in theart, and because they do not further aid in the understanding of thepresent disclosure, a discussion of such components is not provided.

The microwave plasma-generating component 14 includes a microwaveenclosure 24. The microwave enclosure 24 is a rectangular box that ispartitioned using lengthwise sections 26, 28, and 30 having plasma tube32 passing therethrough. Each partition has an opening through which theplasma tube passes. Each section is fed with microwave energy duringoperation. Thus, each section appears to be a relatively short cavity tothe incoming microwave energy, promoting the formation of modes havingazithumal and axial uniformity. Outer tube 34 surrounds the plasma tubeinside the cavity. The outer tube is slightly separated from the plasmatube and air, under positive pressure, is fed between the two tubes toprovide effective cooling of the plasma tube. Tube 32 is preferably madeof sapphire. Other plasma tube materials such as quartz oralumina-coated quartz or other ceramic materials can be used.Preferably, the microwave enclosure 24 is dimensioned to support therectangular TM 110 mode and the enclosure 24 may have a square crosssection. The dimensions of the cross sections are such that the TM 110mode is resonant. The length of each section is less than λg/2 where λgis the guide length within the cavity of the TE 104 mode.

The openings in the partitions 26, 28, and 30 through which theconcentric tubes are fed are made larger than the exterior dimension ofthe plasma tube. Also shown is an iris plate 36, which covers the openside of the microwave structure and is effective to feed microwaveenergy into adjacent sections. Plate 36 is a flat metallic plate havingirises 38, 40, 42, 44, through which the microwave energy is fed. Thereis microwave transmission through these irises, which causes plasma tobe excited in the part of the tube that is surrounded by the partition.Such transmission helps reduce thermal gradients in the plasma tubebetween regions surrounded by partitions and regions that are not. If anouter tube is not used (cooling provided in some other manner) theopenings in the partition are sized so that there is a space between theplasma tube and the partition to provide such microwave transmission.

Microwave traps 46 and 48 are provided at the ends to prevent microwavetransmission. Such traps may be of the type disclosed in U.S. Pat. No.5,498,308. Air seals/directional feeders 50 and 52 are provided foradmitting cooling air and feeding it to the space between the concentrictubes. Air seals/directional feeder 54 are shown at the outlet end and afourth such unit is present but is not seen.

Magnetron 56 provides microwave power that is fed through coupler 58 toa waveguide supplying TE 10 mode, having mutually perpendicular sections60 and 62. The length of waveguide section 62 is adjustable withmoveable plunger 64. The bottom plate of waveguide section 62 is irisplate 36, which couples microwave energy into partitioned microwavestructure 24, through which the plasma tube 32 extends; thus plasma isexcited in the gas mixture flowing through the plasma tube.

Referring again to FIG. 3, it is seen that end cap 70 abuts microwavetrap 48, and fitting 74 having a central orifice for admitting gas tothe plasma tube extends into the end cap. The gas supply 22 is regulatedby an external flow box (not shown). The gas purifier 20 is disposed influid communication with the gas supply 22 and the gas inlet 23 (seeFIG. 1). The plasma tube 32 is supported at this end by “o” ring 72 inthe end cap. The outer tube 34 is supported at its ends by abutmentagainst microwave traps 46 and 48. Spacer 76 is present to provide theproper spacing in relation to the process chamber. The other end of theplasma tube is located in end member 78, and has an opening 80 foremitting plasma/gas into the process chamber 16. Optionally, the conduitforming the opening 80 is fitted with a narrow aperture fitting tocreate a pressure differential between the plasma tube 32 and theprocessing chamber 16, wherein the pressure is greater in the plasmatube 32. During operation, the pressure within the plasma tube 32preferably ranges from about 1 torr to about atmospheric pressure. Incontrast, the pressure within the process chamber during operationranges from about 100 millitorr to about atmospheric pressure.

The opening 80 of the plasma tube 32 is in fluid communication with aninterior region of the process chamber 16. Since the plasma isdischarged from a relatively narrow orifice (compared to the dimensionsof the substrate to be processed) into the interior of the processchamber, a gas distribution system 100 to promote uniform plasmaexposure onto the substrate is disposed within the process chamber 16.The gas distribution system 100 is disposed intermediate to thesubstrate and opening 80 of the plasma tube 32.

In a preferred embodiment, the gas distribution system 100 comprises oneor more baffle plates above the wafer to promote even distribution ofthe plasma to the substrate surface. The baffle plates preferablyinclude multiply stacked baffle plates, wherein each plate contains oneor more apertures. A plenum is formed between the baffle plate assemblyand the upper wall of the process chamber. In an especially preferredembodiment, the baffle plate assembly is adapted to provide more uniformconcentration of reactive species from the plasma to the wafer surface.As discussed in the background section, it has been discovered thathydrogen radicals, for example, created within a plasma decrease inconcentration due to recombination as the hydrogen radicals travel fromthe center most impingement point in the axial flow reactor to the outeredges of the baffle plate. While not wanting to be bound by theory, itis believed that the reduction in activity of hydrogen radicals as thesespecies flow to the outer edges of the baffle plate may be due toshorter lifetimes of hydrogen atoms than can be supported by the radialdistance these species have to travel from the center-fed axial plasmaflow to the outer edges of the plenum. Once the hydrogen radicals haverecombined into molecular hydrogen or the like, the neutral gas can nolonger react with the photoresist. Another reason may be that in anaxial flow reactor design such as the downstream plasma asher describedherein, the photoresist ashing byproducts and spent gas from the centralportions of the wafer must flow past the edge of the wafer in order toreach the exhaust conduit 170 of the process chamber 16. This results insignificant dilution of the active hydrogen radicals nearer the edge ofthe wafer compared to the more central portions and additionallyprovides for the radicals closer to the edge to deactivate by reactingwith the photoresist ashing byproducts that have been removed from thecentral locations. It has been discovered that increased uniformity ofashing can be achieved distally from the centerpoint of the baffle plateto the outer edges by increasing the aperture density of the baffleplate. For example, by increasing the aperture density from thecenterpoint to the outer edges or by increasing the size of theapertures from the centerpoint of the baffle plate to the outer edges,or by including an apertureless centerpoint, or by a combination of oneor more of the foregoing baffle plate configurations can increasereactivity and improve plasma uniformity at the substrate.

FIGS. 5-8 illustrate suitable gas distribution systems for use in theapparatus 10. In a preferred embodiment, the gas distribution system 100is a dual baffle plate assembly. FIG. 5 shows the process chamber 16into which is incorporated a first embodiment of the gas distributionsystem or baffle plate assembly 100. The asher process chamber 16 havingthe baffle plate assembly 100 installed therein is suitable for use in a300 millimeter (mm) wafer processing system. The gas distribution system100 could also be adapted for use with 200 mm wafers, as would beappreciated by one of ordinary skill in the art in view of thisdisclosure. Although the present disclosure is shown as beingimplemented within a downstream plasma asher apparatus, it may also beused in other semiconductor manufacturing equipment, such as residueremoval, stripping, and isotropic etching equipment.

The baffle plate assembly 100 comprises an upper apertured baffle plate102 and a relatively larger lower apertured baffle plate 104 positionedgenerally parallel to each other and separated from one another. Thebaffle plate assembly 100 is attached to a lower portion 106 of theprocess chamber that includes a cavity 108 in which a wafer 110 to beprocessed is placed. The baffle plates 102 and 104, in addition to beingoriented parallel to each other, are also oriented parallel to the wafer110 being processed. The baffle plates 102 and 104 may be the same ordifferent sizes, of may have the same or different number of apertures.In a preferred embodiment, the upper baffle plate 102 has a smallerdiameter than the lower baffle plate 104 as shown in FIG. 5.

A seal 112 is provided at the interface between the baffle plateassembly 100 and the upper portion 106 of the process chamber, andresides within groove 114 in the lower baffle plate 104 (see FIG. 7).Wafers are introduced into and removed from the process chamber via aload lock mechanism (not shown) via entry/exit passageway 116.Alternatively, an atmospheric wafer handling system (not shown) can beemployed to introduce and remove wafers to and from the process chamber.A heater mechanism (discussed below), located under the lower portion106 of the process chamber, heats the underside of the wafer 110 to adesired temperature during processing.

The process chamber 16 is typically installed within the plasma ashingapparatus 10 intermediate to the heater assembly (below) andplasma-generating component 14 (above) at the locations of holes 118.During operation, energized plasma (gas) leaving the plasma tube 32 (seeFIG. 3) encounters the baffle plate assembly 100. In a preferredembodiment, the energized plasma flowing from the plasma tube 32 firstencounters a central area of upper baffle plate 102 that issubstantially free of apertures. This central apertureless area has thefunction of eliminating the high axial gas velocity exiting the plasmatube 32 and accelerating the gas/plasma species in a radial direction inorder to achieve proper operation of the plenum formed between the lowerbaffle plate 104 and the upper wall (i.e., lid) of the process chamber.The plasma is then distributed into the process chamber cavity 108 viaapertures 120 in the upper baffle plate 102 and apertures 122 in thelower baffle plate 104. In one embodiment, the lower baffle plate 104may be actively cooled with a cooling medium flowing through internalcooling passages 124 via inlets 126 and outlets 128. The walls 130 ofthe lower portion 106 of the process chamber may also be actively cooledwith a cooling medium flowing through internal cooling passages 132 viainlet 134 and outlet 136.

The lower baffle plate 104, as shown more clearly in FIGS. 6 and 7,comprises an outer flange 138 and a generally planar portion 140 thatcontains the apertures 122. Mounting holes (not shown) may be providedin the lower baffle plate 16 for mounting the upper baffle plate 1092thereto by means of standoffs 144. The distance between the upper andlower baffle plates in part determines the pattern of gas flow throughthe baffle plate assembly 100. For a 300 mm plasma asher, the distancebetween the upper and low baffle plates, 102, 104, respectively, ispreferably about 0.25 inches to about 2 inches, with a distance of about0.5 to about 1.5 inches more preferred.

FIG. 6 is a plan view of the 300 mm baffle plate assembly shown in FIG.5, and FIG. 7 is a sectional view of this embodiment of the baffle plateassembly 100. As shown in these Figures, the baffle plate assembly 100is mounted to the upper portion 106 of the process chamber via mountingholes 146 in the lower baffle plate flange 138. Apertures 122 areprovided in the lower baffle plate. The surface area of aperturedportion 122 is sufficient to cover the wafer 110 residing therebelow(see FIG. 5). In this embodiment, the size of the apertures 122increases from a centerpoint of the lower baffle plate to an outer edge.The increasing size of the apertures 122 improves plasma uniformity foroxygen and nitrogen free plasma discharges such as for use withcarbon-containing low k dielectrics.

FIG. 8 illustrates a plan view of the lower baffle plate 104 inaccordance with another embodiment. There, the density of the apertures122 increases as one transitions from the center point of the lowerbaffle plate 104 to the outer edge, wherein the sizes of the aperturesare the same.

The apertures 120 in the upper baffle plate 102 are generally arrangedin a radial or concentric multiply circular pattern. The upper baffleplate 102 is comprised of sapphire-coated fused silica or quartz (SiO₂)or a ceramic material. The apertures 120 in the upper baffle plate 102are preferably slightly larger than the largest apertures 122 in thelower baffle plate 104. Located at the center of the upper baffle plate102 is a portion that is free of apertures and my further comprise asapphire impingement disc 152. The center apertureless portion of theupper baffle plate 102, with or without the sapphire impingement disc152, diverts energized gases emanating from the plasma tube 32 radiallyoutward to the remaining apertured area of the upper baffle plate 102,so as to prevent the radially inward portion of the wafer 110 beingprocessed from overheating and over-ashing at a proportionately higherrate than the rest of the wafer due to higher concentration of speciesat about the center. In an alternative embodiment, the upper baffleplate 102 can be configured to be completely apertureless, which may beuseful for processing 200 mm wafers.

Heating of the substrate 110 is preferably accomplished by an array oftungsten halogen lamps 160 (see FIG. 1) positioned below the wafer 110,wherein the wafer is supported by lift pins within the process chamber.A plate 156 (the bottom wall of the process chamber as shown in FIG. 5)transparent to infrared radiation is disposed between the chamber 16 andthe lamps 160. Preferably, the substrate is heated from about 80°centigrade (C) to about 350° C. during ashing. More preferably, thesubstrate is stepwise heated by incrementally increasing thetemperature. Heating has been found to increase the reaction rate of theplasma with the photoresist and/or post etch residues and consequently,increase throughput. The amount of heat applied to the substrate willdepend on the thermal stability of the particular low k dielectric layeras well as the other layers and components already formed in thesubstrate. In a preferred embodiment, the amount of heat is appliednon-uniformly to selected zones of the substrate to facilitate uniformreaction of the plasma with the photoresist. In this embodiment, acontroller (not shown) is in operative communication with the lamp array160 for variously heating the substrate 110 to promote more uniformheating of the wafer during processing. An alternative method of heatingthe wafer is to use a flat heated surface in contact or in closeproximity to heat the wafer, such as a heated chuck.

The substrate 110 is preferably simultaneously exposed to heat ofsufficient intensity and duration, as well as to the nitrogen free andoxygen free plasma so as to cause volatile contaminants to diffuse outof the low-k dielectric layer and volatize without causing degradationof any other components or layers in the substrate. Preferably, forporous or non-porous doped oxide carbon-containing low k dielectricmaterials the wafer is heated from about 20° C. to about 400° C., withabout 100° C. to about 300° C. more preferred. Preferably, for organiclow k materials the wafer is heated from about 80° C. to a maximum ofabout 180° C. The maximum temperatures for organic dielectrics aredependent on the intrinsic properties of the organic low k material usedand can be determined by thermal analysis techniques known to thoseskilled in the art. The temperature may be step-wise increased duringprocessing or remain static.

Openings may also be disposed in the walls 130 of the process chamber 16for purposes generally known in the art such as, for example, an opticalport for monitoring endpoint detection in an in situ chamber cleaningprocess, a mass spectrometer inlet for analyzing gaseous species evolvedduring processing, or the like.

Additionally, the process chamber 16 includes an exhaust opening 158centrally disposed in the bottom plate 156. Preferably, the exhaustopening 158 is coaxial with the plasma tube 32.

The operating pressures within the process chamber 16 are preferablyabout 100 millitorr to about 3 torr, with about 200 millitorr to about 2torr more preferred, and with about 500 millitorr to about 1.5 torr evenmore preferred. Moreover, the process chamber 16 may further includeadditional features depending on the application. For example, a quartzwindow may be installed and a UV light source may be placed in proximityto the wafer. Such a non-columnar light source may have a wavelengthsimilar to UV excimer lasers that have been shown to enhance photoresistremoval in bulk strip applications and as such, could be used inparallel with microwave plasma generated reactive gases. Moreover, pre-and post-photoresist strip exposure to the light source could alsoprovide residue removal and implanted resist removal advantages.Overhead RF sources, optical ports, gas analyzers, additional lightsources, and the like could also be used either independently, or incombination, with the process chamber 16 providing an extremely flexibleprocess platform.

Coupled to the process chamber 16 is the exhaust assembly component 18.The exhaust assembly component 18 includes an exhaust conduit 170 influid communication with an interior region of the process chamber 16.An inlet 172 of the exhaust conduit 170 is fluidly attached to opening158 in the bottom plate 156 of the process chamber 16. The exhaustconduit 170 preferably has a substantially straight shape from inlet 172to outlet 174, thereby minimizing high impact areas (e.g., sharp bendsand curves in the conduit) and the propensity for buildup of photoresistmaterial and plasma ashing byproducts at sharp bends. In a preferredembodiment, the exhaust conduit 170 is fabricated from quartz orsapphire coated quartz. A minimum diameter of the exhaust conduit 170(and opening 156) is preferably at least about 2 inches for a 300 mmashing apparatus (about a 1.5 inch diameter or greater is preferred fora 200 mm plasma ashing apparatus). By centrally locating the exhaustconduit 170 within the process chamber 16, flow from the plasma tube tothe exhaust assembly is simplified and provides greater plasmauniformity.

The outlet 174 of the exhaust conduit 170 is preferably connected tovacuum system 176. An afterburner assembly 178 is in operativecommunication with the exhaust conduit 170. A gas inlet 180 and gassource 182 are in fluid communication with the exhaust conduit 170 andare positioned upstream from the afterburner assembly 178. Theafterburner assembly 178 is employed to generate a plasma dischargewithin the exhaust conduit 170 so as to volatilize photoresist materialand plasma byproducts discharged from the process chamber 16. As will bedescribed in greater detail below, the gas source 180 is preferably anoxidizing gas such as oxygen or a combination of gases includingoxidizing gases. Preferably, the oxidizing gas is free of halogens. Inthis manner, effluent from the process chamber into the exhaust conduitis mixed with the oxidizing gas source e.g., oxygen, and an oxygencontaining plasma is formed from the mixture by the afterburner assembly178, the manner of which is described below. It is preferred that theoxidizing gas is introduced to the afterburner assembly immediatelyabove the assembly and is downstream from the exhaust opening 158 of theprocess chamber 16. Entry of oxygen into the process chamber 16 candeleteriously affect the low dielectric material in the mannerpreviously described. The hardware and process for generating plasma inthe exhaust conduit is preferably adapted to prevent oxygen fromtraveling upstream, i.e., back into the process chamber.

The afterburner assembly 178 preferably comprises an RF coil 183 wrappedabout an exterior of the exhaust conduit 170 to inductively excite a gasmixture flowing through the exhaust conduit 170. Although reference ismade to inductively coupling the gas mixture with RF power to form theplasma, other means could be employed in an effective manner such as bycapacitive excitation or the like. Additionally, other frequencies inthe ISM band including microwaves may be used to excite the afterburnerplasma. The oxidizing gas is preferably introduced at inlet 180 upstreamfrom the afterburner assembly 178. A throttle valve 184, foreline valve(not shown), vacuum pump 176, and other vacuum processing lines aredisposed downstream from the afterburner assembly 178.

The RF coils 182 are connected to a suitable RF generator or powersupply 186. The power supply frequency may vary, typically ranging from400 KHz to the preferred value of 13.56 MHz at less than 600 watts (W),but may also be at higher frequencies and higher power. More preferably,an RF power of about 300 W to about 500 W is employed to inductivelycouple an oxygen species containing plasma in the exhaust conduit 170,which causes the organic matter contained therein to combust. As aresult, deposition of photoresist material and other organic byproductsdownstream from the process chamber is prevented and/or removed.

The RF connections are typically made through an RF matchbox 188 and thecoils 182 are energized at the beginning of the plasma ashing process.The oxygen containing (O₂) gas admixture passing through the coupled RFfield produces a plasma discharge that effectively and efficientlycombusts organic matter. Preferably, the afterburner assembly 178 isconfigured to simultaneously operate during plasma ashing processing ofa substrate 110 in the process chamber 16.

Additionally, the exhaust conduit 170 also includes an optical detectionsystem 190. The optical detection system 190 optically detects emissionpeaks having particular wavelength ranges that correspond to thereaction byproducts (and reactants) of the reactions between the plasmaand the photoresist. The technique relies on detecting the change in theemission intensities of characteristic optical radiation from thereactants and byproducts in the plasma. Excited atoms or molecules emitlight when electrons relax from a higher energy state to a lower energystate. Atoms and molecules of different chemical compounds emit a seriesof unique spectral lines. The emission intensity for each chemicalcompound within the plasma depends on the relative concentration of thechemical compound in the plasma. A typical optical emission spectroscopyapparatus operates by measuring the emission intensities of the reactiveetching gas and the by-product of the etching gas and the reactants. Theemission decreases and finally stops when the byproduct is no longer inthe viewing location, and an endpoint is reached. The optical emissionspectroscopy apparatus senses the declining emission intensity of theby-product to determine this endpoint. Alternatively, the opticalemission spectroscopy apparatus can sense the rise in reactant speciesonce an endpoint is reached in the process chamber, such that either arise in reactants or conversely, a fall in product emissions may be usedto trigger endpoint. Advantageously, optical signals downstream from thedischarge region of the afterburner assembly 178 can be used to clearlyindicate what is occurring at the wafer surface in the process chamber.For example, an oxidizing agent such as oxygen is consumed when theplasma is ignited within the exhaust conduit 170 and combustion productsare generated. The combustion products, e.g., carbon monoxide, carbondioxide, water and the like, are those typically encountered duringplasma ashing of photoresist with oxygen containing plasma discharges,but not typically from an oxygen free and nitrogen free plasmadischarge. Since these species emit strong optical emission signals, anoxygen free and nitrogen free plasma process can be readily monitoredfor endpoint detection by analyzing the optical signals produced fromthe afterburner assembly in the exhaust conduit plasma discharge region.Once the signal of the monitored species is undetectable, it can bepresumed that endpoint has been reached. As previously discussed, theuse of oxygen free and nitrogen free plasma processes are desired forremoving photoresist masks and the like from substrates containingcarbon-containing low k dielectrics. Suitable oxygen free and nitrogenfree plasma processes for use in the present disclosure are disclosed inpending U.S. patent application Ser. No. 09/855,177, to Waldfried etal., incorporated herein by reference in its entirety. The presentprocess and apparatus provides a means for endpoint detection, whichotherwise is generally difficult to directly detect in a process chamberin view of the species generated during an oxygen free and nitrogen freeplasma ashing process.

The optical detection system 190 is coupled to the exhaust conduit.Collection optics 192 may be arranged outside the exhaust conduit 170 tocollect the emission spectra thus passed, looking directly into theplasma generation region through the RF coils 182. Since the exhaustconduit 170 is preferably fabricated from an optically transparentmaterial such as quartz or sapphire, an optical port or window is notnecessary. In the event that an optically non-transparent material isemployed for the fabrication of the exhaust conduit, an optical port ofquartz or sapphire may be formed in the exhaust conduit. A spectrometeror monochromator (generally shown as 194 in FIG. 1) is arranged toreceive light from the collection optics 192. Optical emissionspectroscopy and techniques are generally well known on the art. In oneembodiment, the optical emission spectroscopy is by a spectrometer, suchas a CCD (charge couple device) based spectrometer or a PDA (photodiodearray) based spectrometer, that time sequentially records a wavelengthrange and converts the emission spectra into analog signals forsubsequent analysis. Optionally, narrow band filters can be used topermit evaluation of specific ranges of the wavelength of interest on alight detector such as a photomultiplier tube (PMT) or a photodiode. Thespectrometer time sequentially converts light signals emitted during thecombustion process in the afterburner assembly at specific wavelengthsinto an electrical analog signal, which can then be analyzed usingmethods known to those skilled in the art to produce a desired output.Preferably, the data is viewed in real time. Preferably, the data isviewed in graphical form showing the time evolution of the lightintensity emitted during plasma processing for the wavelength range ofinterest. Additionally, the drop (or rise depending on the speciesmonitored) in the optically induced analog signal can be used to triggerevents on the machine. For example. Upon determining ashing endpoint hasoccurred from data collected by the optical detector in the exhaustconduit, the plasma ashing process can be immediately discontinued via afeedback loop.

Alternatively, other optical detectors can be used. For instance, asdiscussed above, a monochromator can be used to collect the data. As isknown to those skilled in the art the monochromator can be configuredwith a photomultiplier tube, a photodiode or the like to record theemission signal.

These optical emission spectroscopy devices and suitable configurationswithin a plasma reaction chamber will be apparent to those skilled inthe art in view of this disclosure. An example of a monochromatorsuitable for use in the present disclosure is model no. EP200MMDcommercially available by the Verity Corporation. An example of ascanning monochromator suitable for use in the present disclosure ismodel no. EP200SMD commercially available by the Verity Corporation.Examples of CCD based spectrometers suitable for use in the presentdisclosure are Model Nos. SD1024 commercially available by VerityCorporation, and series PC2000 CCD spectrometers commercially availablefrom Ocean Optics. An example of a photodetector array suitable for usein the present disclosure is model no. SPM9001 commercially availablefrom the Prema Company, Germany.

Preferably, the endpoint detection process and apparatus is used withsubstrates, wherein the low k materials contain carbon and/or hydrogenwithin its structure such as doped oxides, porous materials and organiclow k films. The carbon-containing low k dielectric materials mayinclude pendant groups that contain carbon or may be carbon containingwherein the backbone of the dielectric material is primarily comprisedof an interconnecting network of carbon. The process employing thenitrogen-free and oxygen-free plasma provides high ashing selectivityand overcomes the problems noted in the prior art that occur from ashingphotoresist, polymers and residues from carbon and/or hydrogen based lowk dielectric materials. Moreover, the process alleviates the subsequentmetal filling problems caused by nitrogen in the ashing plasma.

The ashing process includes generating reactive species from a plasmagas mixture and exposing a substrate to the reactive species. Theparticular components of the plasma gas mixture are selected by theirability to form a gas and plasma under plasma forming conditions. Thegas mixture selected is free from components that generate reactiveoxygen species and reactive nitrogen species under plasma formingconditions. More preferably, the gas mixture is free fromoxygen-containing compounds and nitrogen-containing compounds. The gasmixture may include a number of reactive gases that are hydrogen-bearinggases, e.g., hydrogen gas, hydrocarbon gases, and the like. The gasmixture may further comprise an inert gas such as argon, helium, neonand the like. The plasma generated from the gas mixture primarily reactswith carbon and other atoms in the photoresist, polymers, and residuesto form compounds that are volatile under the temperature and pressureconditions at and about the substrate and/or rinse removable compounds.The process is optimized to have a selectivity greater than 50:1.

Hydrogen-bearing gases suitable for use in the process include thosecompounds that contain hydrogen. The hydrogen-bearing gases includehydrocarbons, hydrogen gas or mixtures thereof. Preferredhydrogen-bearing gases exist in a gaseous state under plasma formingconditions and release hydrogen to form reactive hydrogen such as atomichydrogen species and other hydrogen radicals under plasma formingconditions. The hydrocarbons are preferably unsubstituted. Examples ofsuitable hydrogen-bearing hydrocarbon gases include methane, ethane andpropane.

Preferred hydrogen-bearing gases are mixtures of a hydrogen bearing gasand a noble gas. Examples of noble gases suitable for use in the processinclude a gas in Group VIII of the periodic table such as argon, neon,helium and the like. Although prior art oxygen-free plasma dischargesgenerally use a forming gas composition that includes a hydrogen andnitrogen gas mixture, the use of nitrogen gas in the process isexpressly excluded. Consequently, since forming gas is hereinafterdefined as a gas containing a mixture of hydrogen and nitrogen gases,the use of forming gas in the process is expressly excluded.Particularly preferable for use in the present invention is a gasmixture that includes hydrogen and helium gases. Helium gas atoms arelight and readily diffuse to the substrate, which results in excellentcarrier characteristics for plasma generated reactive hydrogen species.

For safety reasons, the percentage of hydrogen gas in the gas mixturegenerally does not exceed about 5 percent by volume of the gas mixture.However, higher amounts of hydrogen are acceptable and sometimespreferred for increasing the photoresist removal rate and selectivity.Preferably, the amount of hydrogen in the gas mixture is from about 1 toabout 99 percent of the total volume. More preferably, the amount ofhydrogen in the gas mixture is from about 10 to about 30 percent of thetotal volume.

In operation, the semiconductor wafer 110 with photoresist and/or postetch residues thereon (and a carbon-containing low k dielectricmaterial) is placed into the process chamber 16 on wafer support pins.The wafer 110 is preferably heated by infrared lamps 160 to acceleratethe reaction of the photoresist and/or post etch residues with theplasma. The pressure within the process chamber 16 is then reduced.Preferably, the pressure is maintained between about 1 torr to about 5torr. An excitable oxygen free and nitrogen free gas mixture is fed intothe purifier and then the plasma tube 32 of the plasma-generatingcomponent 14 via a gas inlet 24. Each section 26, 28, 30 of the plasmagenerating component 14 is fed with microwave energy to excite a plasmain the plasma tube 32, which plasma is comprised of electrically neutraland charged particles. The charged particles are preferably selectivelyremoved before the plasma enters the process chamber 16. The excited orenergetic atoms of the gas are fed into the process chamber anduniformly distributed across the wafer where the atomic hydrogen reactswith the photoresist and/or post etch residues, which causes removal ofthe photoresist material and also forms volatile byproducts. Thephotoresist material and volatile byproducts are continuously swept awayfrom the wafer surface to the centrally located exhaust conduit 170.

Simultaneously with plasma ashing, an oxidizing gas is fed into theexhaust conduit 170 downstream from the process chamber 16. No oxygenenters the process chamber 16 due to the “plug-flow” condition imposedby the much larger helium hydrogen flow rate from the process chamberinto the exhaust conduit 170. The afterburner assembly 178 is energizedto form high-density plasma within the exhaust conduit 170. For anexhaust conduit 170 configured with a CCD based spectrometer, the CCDspectrometer time sequentially records an emission spectrum thatincludes emission signals corresponding the photoresist material andvolatile byproducts, if present. The wavelength range of the emissionspectrum monitored is determined by the type of CCD spectrometer usedand the presence of any filters used to eliminate certain wavelengthemissions from reaching the CCD spectrometer. The CCD spectrometerconfiguration simultaneously records the background radiation and theradiation from the emitted species during the ashing process. Usingstandard algorithms known to those skilled in the art, the backgroundradiation can be subtracted from the radiation resulting from thereaction of the plasma with the photoresist and/or byproducts. Once theemission peak records a change in intensity values and the conditionsset by an endpoint algorithm are met, the removal of photoresist and/orresidues is complete, a signal is then sent to a control unit and theplasma can be turned off. The vacuum is released and the processedwafers may be removed from the process chamber. An optional water rinseis used to remove any remaining residue on the stripped wafer.

In a plasma asher with a monochromator, blank uncoated wafers are firstexposed in the process chamber 16 and a first emission signal at adesired wavelength is measured in the exhaust conduit 170. The firstemission signal represents the background radiation as discussed above.Next, substrates having photoresist and/or residues thereon (andcontaining a carbon-containing low k dielectric material) are exposed toplasma in the process chamber. A second emission signal emitted in atthe desired wavelength is recorded in the exhaust conduit by themonochromator. The background radiation of the first emission signal issubtracted from the second emission signal. When the second emissionsignal at the desired wavelength reaches a steady state and is about thesame or below the first emission signal, the ashing endpoint has beenreached in the process chamber 16 and a signal is then sent to a controlunit in the plasma asher 10 and the plasma is turned off. The vacuum isthen released and the processed wafers are removed from the processchamber. An optional water rinse is then used to remove any remainingresidues on the stripped wafer.

Other monochromators, spectrometers or like configurations andoperations thereof for monitoring the plasma byproducts discharged fromthe process chamber will be apparent to those skilled in the art in viewof this disclosure. Preferably, one or more of the emission signals at283 nm, 309 nm, about 387 nm, about 431 nm, about 434 nm, about 468 nm,about 472 nm, about 513 nm, about 516 nm, about 656 nm, about 668 nm,about 777 nm, about 845 nm (+about 5 to about 10 nm) are monitored inthe exhaust conduit 170. These emission signals represent spectral peaksfor photoresist materials, reactants, and plasma byproducts formed by amixture of photoresist components, the nitrogen free and oxygen freeeffluent from the process chamber, and the oxygen plasma dischargegenerated by the afterburner assembly. For example, so called “Swanbands” correlating to the dimer C2 are evident at about 513 nm and about517 nm. Upon addition of an oxidizing gas in the exhaust conduit,emission signals from CO/CH species at about 431 nm and CN species atabout 387 nm can be readily monitored. Since most I-line photoresistsare based on diazonapthoquinone chemistry, monitoring the emissionsignal intensity for CN is quite useful for ashing endpoint detection ofl-line photoresists. Moreover, the intensity of the emission signals canincrease or decrease within the exhaust conduit, which can be also usedto determine endpoint of the ashing process in the process chamber. Forexample, during plasma ashing of photoresist in the process chamber, theintensity can increase for emissive species correlating to H at about434 nm and at about 656 nm, OH at about 283 nm and 309 nm, and 0 atabout 777 nm as ashing of the photoresist nears completion. In thismanner, an oxygen free and nitrogen free plasma ashing process can beused to remove the photoresist material and post etch residues fromsubstrates containing carbon-containing low k dielectrics whereas asecond oxidizing plasma is formed in the exhaust conduit 170 todetermine ashing endpoint for the oxygen free and nitrogen free plasmaby monitoring the emission signals of the reactants and/or products inthe exhaust conduit 170.

Unless otherwise specified, the materials for fabricating the variouscomponents 12, 14, 16, and 18 include metals, ceramics, glasses,polymers, composite materials, and combinations comprising at least oneof the foregoing materials. For example, suitable metals includeanodized aluminum, and/or stainless steel. Suitable ceramic materialsinclude silicon carbide, or aluminum oxide (e.g., single crystal orpolycrystalline).

The following examples are presented for illustrative purposes only, andare not intended to limit the scope of the disclosure.

Example 1

In this example, optical signals for OH and H were monitored in theexhaust gas conduit downstream from an afterburner in a plasma ashingapparatus as shown in FIG. 1. Helium was introduced into the plasmaapparatus at a flow rate of 7,000 sccm and at a pressure of 1.5 torr. Asubstrate containing photoresist was exposed to heating lamps to slowlyheat the wafer to 300° C. and volatilize organic matter from the wafersurface. No plasma was created in the process chamber at this time.During operation of the afterburner at an RF power of 500 W, oxygen gaswas introduced into the exhaust conduit at a flow rate of 1,000 sccm. Nooxygen was introduced into the process chamber. The optical signals forOH and H were monitored in the exhaust conduit over a period of time todetermine endpoint of the plasma ashing process in the process chamber.FIG. 9 graphically illustrates light intensity for these optical signalsas a function of time. It was observed that as the organics sublimedfrom the wafer were oxidized in the afterburner. Since different organicspecies sublimate at different temperatures, the multiple peaks shown inFIG. 9 correspond to different temperature settings. If plasma were usedin the process chamber simultaneously with the wafer heating, it isbelieved that neither of these signals (corresponding to H and OH) couldbe employed for endpoint detection in the process chamber since there isno emission of consequence in the process chamber for these species.

Example 2

In this example, CO₂ was monitored in an exhaust conduit of a plasmaasher apparatus as shown in FIG. 1. CO₂ was monitored by a residual gasanalyzer with and without plasma formed by an afterburner disposed inthe exhaust conduit. Similar to Example 1, a resist coated wafer wasslowly heated to 300° C. in the process chamber without exposure toplasma. Helium was introduced into the plasma apparatus at a flow rateof 7,000 sccm and at a pressure of 1.5 torr. During operation of theafterburner at an RF power of 300 W, oxygen gas was introduced into theexhaust conduit at a flow rate of 1,000 sccm. No oxygen was introducedinto the process chamber. FIG. 10 graphically illustrates CO₂ generationas a function of time resulting from generating plasma in the exhaustconduit. If the afterburner was not used, no detectable CO emissionwould result. However, exposing the organics from the process chamber tothe afterburner resulted in strong emission of CO It is noted thatwithin the 2′ process chamber, no signal would be emitted from CO evenif the wafer were exposed to the oxygen free and nitrogen free plasma,e.g., plasma formed from a helium and hydrogen gas mixture.

Example 3

In this example, dilution tests were performed to determine the minimumupstream helium flow rates to prevent backstreaming of the oxygen gasinto the process chamber. Oxygen was flowed at a rate of 1,000 sccm intothe exhaust conduit. A helium gas was flowed into the plasma apparatusinitially at a flow rate of 7,000 sccm and was stepwise decreased.Residual gas analysis was taken upstream of the afterburner to monitorpartial pressures of helium, nitrogen, and oxygen. FIG. 11 graphicallyillustrates partial pressures of helium, nitrogen and oxygen as afunction of time and dilution. At a helium flow rate of about 175 sccm,it is observed that oxygen is backstreaming into the upstream residualgas analyzer, which could potentially be detrimental for plasma ashingcarbon-containing low k dielectric.

Example 4

In this example, a resist coated wafer is heated slowly, with 7 standardliters per minute (slm) of helium flow in the chamber, and 1 slm of O₂flow in the side-feed of the afterburner. Time evolution of the opticalsignals for the reactants (O, CN) and the product (OH) are observed. Asthe wafer begins to heat, volatile byproducts sublimate and are consumedin the afterburner. The OH signal rises to show this, with acorresponding drop in the 0 signal. Also, carbon, which was being usedto create CN, is now used to make CO and CO₂, with a corresponding dropin the CN signal.

Example 5

In this example, 6,000 angstroms of DUV photoresist was coated ontowafers and treated in a plasma ashing apparatus similar to the one asshown and described with reference to FIG. 1. A 4% hydrogen and 96%helium gas mixture (percentages by volume) at a flow rate of 10 slm wasintroduced into the plasma tube from which plasma was generated. Thewafer was exposed to the plasma in the process chamber and the effluentproduced therein was discharged from the process chamber into theexhaust conduit. Wavelengths correlating to CO/CH (431 nm), C2 dimer(517 nm), H (656 nm), and He (668 nm) species were monitored in theexhaust conduit after passing through the energized afterburner. Theresults are shown in FIG. 13.

As shown, the hydrogen/helium plasma ashing process took about 30seconds to remove the photoresist material. Hydrogen and helium speciesincreased as a function of time indicating that these species weredecreasingly involved in the plasma ashing removal of the photoresistmaterial in the process chamber. In contrast, the C2 dimer and CO/CHemissive species decreased as a function of time since these species areno longer generated the wafer is clear of photoresist. Any one or acombination of the species monitored can provide a robust endpointmeasurement process.

Example 6

In this example, 6,000 angstroms of DUV photoresist was coated onto thewafers and treated in a plasma ashing apparatus as in Example 5. Oxygenat a flow rate of 1 slm was introduced into the exhaust conduit (i.e.,afterburner assembly but not in the process chamber) to produce anoxidizing plasma in the exhaust conduit with the oxygen free andnitrogen free plasma effluent produced in the process chamber by thehydrogen/helium ashing process. Wavelengths correlating to CO/CH (431nm), C2 dimer (517 nm), and OH (309 nm) species were monitored in theexhaust conduit after passing through the energized afterburner. In thisexample, dummy wafers (no photoresist material) were also exposed to thesame process. The results are shown in FIG. 14.

At about 20 to about 110 seconds, no change in emission intensities wasobserved with the dummy wafers. At about 185 to about 270 seconds, achange in emission signal intensities is observed with the coated wafersas the photoresist is stripped therefrom.

Example 7

In this example, an l-line photoresist coated at a thickness of 1.8microns was processed in the plasma ashing apparatus similar to the oneas shown and described with reference to FIG. 1. A 4% hydrogen and 96%helium gas mixture (percentages by volume) at a flow rate of 10 slm wasintroduced into the plasma tube from which plasma was generated. Oxygenwas introduced into the exhaust conduit at a flow rate of 1 slm.Wavelengths correlating to CO/CH (431 nm), C2 dimer (517 nm), 6 (777 nmand 845 nm), and H (434 nm), species were monitored in the exhaustconduit after passing through the energized afterburner. The results areshown in FIG. 15.

Reference to EP and EP2 refer to an algebraic manipulation of the 4traces to provide greater signal to noise ratio. As shown, endpointdetection can be accurately determined using individual wavelengths orby providing an algorithm of the four traces. Here, the photoresist wasremoved after a plasma ashing process of about 100 seconds.

While the disclosure has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method for detecting an endpoint for an oxygen free and nitrogenfree plasma ashing process, comprising: exposing a substrate comprisingphotoresist material and/or post etch residues thereon to the oxygenfree and nitrogen free plasma in a process chamber; removing thephotoresist material and/or post etch residues from the substrate;exhausting the removed photoresist material and/or post etch residuesfrom the process chamber into an exhaust conduit fluidly coupled to theprocess chamber; selectively introducing an oxidizing gas into theexhaust conduit; generating a plasma from the oxidizing gas and theexhausted photoresist material and/or post etch residues to formemissive species; and optically monitoring an emission signal producedby the emissive species to determine the endpoint of the oxygen free andnitrogen free plasma ashing process.
 2. The method of claim 1, whereinthe emission signal produced by the emissive species is monitored at oneor more wavelengths of about 283 nm, about 309 nm, about 387 nm, about431 nm, about 434 nm, about 468 nm, about 472 nm, about 513 nm, about516 nm, about 656 nm, about 668 nm, about 777 nm, and/or of about 845nm.
 3. The method of claim 1, wherein exposing the substrate to theoxygen free and nitrogen free plasma in the process chamber andgenerating the oxygen plasma in the exhaust conduit occurssimultaneously.
 4. The method of claim 1, wherein the substratecomprises a carbon and/or hydrogen containing insulating layer having adielectric constant less than about 3.5.
 5. The method of claim 1,wherein the an oxygen free and nitrogen free plasma ashing processcomprises generating the oxygen free and nitrogen free plasma from a gasmixture comprising hydrogen or helium or a combination comprising atleast one of the foregoing gases.
 6. The method of claim 1, furthercomprising preventing backstreaming of the oxidizing gas into theprocess chamber.
 7. An endpoint detection process for an oxygen free andnitrogen free plasma ashing process for removing photoresist and/orresidues from a substrate, comprising: introducing an oxidizing gas anda plasma ashing discharge into an exhaust conduit of a plasma asherapparatus, wherein the plasma ashing discharge comprises photoresistmaterial, post etch residues, and post ashing products, and wherein theplasma ashing discharge is free from nitrogen and oxygen species;generating a plasma from the oxidizing gas and the plasma ashingdischarge to form emissive species; and optically monitoring emissionsignal intensities correlating to the emissive species, wherein anendpoint of the oxygen free and nitrogen free plasma ashing process isdetected when the emission signal intensities correlating to theemissive species substantially change to an amount greater or less thana predetermined threshold.
 8. The endpoint detection process of claim 6,wherein the emission signal intensity is at a wavelength selected fromthe group consisting of about 283 nm, about 309 nm, about 387 nm, about431 nm, about 434 nm, about 468 nm, about 472 nm, about 513 nm, about516 nm, about 656 nm, about 668 nm, about 777 nm, about 845 nm, and acombination of at least one of the foregoing wavelengths.
 9. Theendpoint detection process of claim 6, wherein the substrate comprises acarbon and/or hydrogen containing insulating layer having a dielectricconstant less than about 3.5.
 10. The endpoint detection process ofclaim 6, wherein the oxidizing gas comprises oxygen.
 11. The endpointdetection process of claim 6, wherein optically monitoring the emissivespecies comprises focusing collection optics of an optical detector ator about a plasma discharge region for the plasma from the oxidizing gasand the plasma ashing discharge.
 12. A method for determining anendpoint of an oxygen free and nitrogen free plasma ashing process usedfor stripping photoresist material from a substrate having acarbon-containing low k dielectric material, comprising: exposing thesubstrate to the oxygen free and nitrogen free plasma ashing process ina process chamber to remove the photoresist material from the substrateand form volatile byproducts; exhausting the photoresist material andvolatile byproducts from the process chamber into an exhaust conduit;selectively introducing an oxidizing gas into the exhaust conduit,wherein the oxidizing gas does not flow into the process chamber;generating a plasma in the exhaust conduit from the oxidizing gas, theexhausted photoresist material, and the volatile byproducts; measuringan emission signal intensity in the exhaust conduit correlating to awavelength selected from the group consisting of about 283 nm, 309 nm,about 387 nm, about 431 nm, about 434 nm, about 468 nm, about 472 nm,about 513 nm, about 516 nm, about 656 nm, about 668 nm, about 777 nm,about 845 nm, and a combination of at least one of the foregoingwavelengths; and determining the endpoint of the oxygen free andnitrogen free plasma ashing process in response to an observed change inthe emission signal within the exhaust conduit.
 13. A method fordetermining an endpoint of an oxygen free and nitrogen free plasmaashing process used for stripping photoresist material and/or residuesfrom a substrate having a carbon-containing low k dielectric material,comprising: generating a first plasma in a process chamber in theabsence of oxygen and nitrogen from a gas mixture comprising hydrogen orhelium or a combination comprising at least one of the foregoing gases;exposing the substrate provided in the process chamber to the firstplasma to selectively remove photoresist material and/or residues fromthe substrate; exhausting the removed photoresist material and/orresidues from the process chamber into an exhaust conduit; generating asecond plasma in the exhaust conduit to generate emissive species; andoptically monitoring the emissive species, wherein an endpoint of thefirst plasma is detected when an intensity of the emissive specieswithin the second plasma changes to an amount greater or less than apredetermined threshold.
 14. The method of claim 12, wherein generatingthe second plasma comprises introducing an oxidizing gas into theexhaust conduit, wherein the oxidizing gas does not flow into theprocess chamber.
 15. The method of claim 12, wherein the second plasmais free of an oxidizing gas.
 16. The method of claim 12, whereinoptically monitoring the emissive species comprises monitoringwavelengths of the emissive species correlating to reactant speciespresent in the process chamber.
 17. The method of claim 12, whereinoptically monitoring the emissive species comprises monitoringwavelengths of the emissive species optically monitoring speciesproduced by a reaction between the second plasma and the removedphotoresist material and/or residues.